It is desired that a sliding-contact surface of a certain member, with which another certain member contacts slidingly, be excellent, for example, in the abrasion resistance, the toughness, and the strength. In order to improve various characteristics as described above, for example, it has been hitherto suggested that various surface treatments including the carburizing, the sulfurizing, the nitriding, and the carbonitriding are applied to a member composed of stainless steel. Any of the treatment methods is carried out while keeping the member at a high temperature.
A passive film composed of iron oxide, which is spontaneously generated by being oxidized by oxygen contained in the air, is present on the surface of the iron group alloy including stainless steel. For example, the passive film inhibits the progress of the nitriding process when the stainless steel is subjected to the nitriding treatment. As a result, the nitriding efficiency tends to be lowered.
In view of the above, the removal of the passive film is widely carried out before applying the nitriding treatment. The wet method has been hitherto adopted as the removing treatment, in which the stainless steel is immersed in an aqueous solution of cyanogen compound or the like. However, the cyanogen compound is toxic. Therefore, an inconvenience arises such that handling the compound is a burden on the operator, and a detoxifying mechanism is required to be installed. Further, the waste liquid after use needs to be treated by means of an appropriate method.
In order to avoid the inconvenience as described above, the so-called dry method such that the passive film is removed by means of the mechanical polishing, or the heat treatment is performed in a reaction gas atmosphere containing fluorine, is suggested in Japanese Laid-Open Patent Publication Nos. 5-263278 and 7-54123 respectively. However, in the case of the method described in Japanese Laid-Open Patent Publication No. 5-263278, it is difficult to allow a workpiece to have a desired shape. On the other hand, in the case of the method described in Japanese Laid-Open Patent Publication No. 7-54123, it is necessary to install a detoxifying mechanism, because the fluorine gas is a toxic material. Therefore, an inconvenience arises such that cost of facility and equipment is high. Further, it is hard to say that the working environment is safe, because the toxic material is used.
In view of the above, it is conceived that the hydrogen sputtering method, which is one type of the dry method, is adopted as suggested in Japanese Patent Publication No. 2-2945. In this procedure, a mixed gas composed of hydrogen gas and nitrogen gas is introduced into a treatment chamber to cause the glow discharge in which the treatment chamber is the anode, an auxiliary electrode is the cathode, and a workpiece is the neutral. The passive film is reduced and removed by hydrogen ion and ammonia ion generated thereby.
However, the method described in Japanese Patent Publication No. 2-2945 involves the following inconvenience. That is, the depths of permeation and diffusion of hydrogen ion and ammonia ion with respect to the workpiece are small in a certain type of steel material such as steel materials having high contents of Cr, Ni, or the like. For this reason, the passive film is insufficiently removed. Therefore, the compound layer, which is to be formed by the plasma nitriding process thereafter, may have nonuniform thicknesses, and some portions, at which no compound layer is formed, may disadvantageously be generated in some situations.
A general object of the present invention is to provide a surface treatment method for a metal material, which makes it possible to easily and conveniently remove a passive film on a surface of the metal material irrespective of the quality of the metal material and which can be carried out in a safe environment.
A principal object of the present invention is to provide a surface treatment method for a metal material, which makes it possible to continuously carry out various surface treatments such as the nitriding and the carburizing after removing a passive film.
Another object of the present invention is to provide a surface treatment method for a metal material, which makes it possible to improve hardnesses of various metal materials to deeper inside portions thereof and which can be carried out easily and conveniently in a safe environment.